This application relates to semiconductor devices, and more particularly, to high voltage power metal-oxide-semiconductor field-effect transistors (MOSFET).
One of the most important components in a high voltage semiconductor power MOSFET is the lightly doped drain (LDD) region (also known as “drift” region). While LDD helps a device supporting high applied voltage during the voltage blocking “off” state, it also heavily limits the current conductivity during “on” state then current is conducting. In a conventional high voltage power MOSFET, due to the LDD resistance, the total on-resistance (Ron) rapidly increases with the desired reverse breakdown voltage (Vb). The relationship can be roughly expressed as a function of Ron˜Vb3. For example, in a 750V conventional vertical MOSFET, the LDD contributes almost 99% of the device's total on-resistance.
There have been a few methods developed over the years to reduce the on-resistance. They help continually driving down device unit area on-resistance, or specific on-resistance. Most methods attempt to increase LDD doping density for a given reverse breakdown voltage. One of the examples is reduced surface field (RESURF), which utilizes partial charge compensation or conductor field plates to reduce peak electrical field, it allows a higher total LDD doping profile to be used (J. A. Appels and H. M. J. Vaes, HV Thin Layer Devices (RESURF Devices), Proc. Intl. Electron Devices Meeting, pp. 238-241, 1979). Another method introduces the so-called charge balance (CB) to help deplete LDD before its breakdown (Coe, U.S. Pat. No. 4,754,310A; Chen, U.S. Pat. No. 5,216,275; and Tihanyi, U.S. Pat. No. 5,438,215). Charge balance method has been mainly used in vertical MOSFET and over the years it has gradually reduced specific on-resistance. The RESURF and field control methods are more widely used in lateral MOSFET that allow easy integration with lower voltage devices. Over the years, they have gradually evolved into so-called multiple RESURF by combining charge balance effects. Charge balance and multiple RESURF are currently producing the best specific on-resistance Rs in integrated MOSFET. They rely on columns of P and N type materials that deplete each other when biased into off-state. In general, doping density, and thus conductivity, can be increased when column width is reduced. But they are relatively difficult to mass produce, due to the difficulty in both column width and doping density control, especially the latter that requires P and N types be closely matched. Due to these limitations, multiple RESURF method finds only limited use in production.
In pursuing the same goal of more effectively depleting LDD, other field manipulating techniques have been proposed. But in general, they are much less successful. For example, in Kocon, U.S. Pat. No. 6,717,203, bias electrodes disposed adjacent the drift (LDD) region to alter the electric field in the drift region to increase reverse breakdown voltage. This approach is relatively complex, and the extra controlling terminals make it difficult to be implemented in applications. In Darwish, U.S. Pat. No. 8,592,906, capacitively coupled floating electrodes in LDD provide a similar means to deplete the LDD with increasing applied voltage, but it has the disadvantage of not being stable enough due to the presence of floating components.
There is therefore a need for an improved advanced on art to have an improved semiconductor high voltage power MOSFET that is simple to implement and provides control stability.